A mat shot is a tongue-and-cheek type of shot where the bartender grabs a shot glass, picks up the bar service spill mat in front of them and pours the entire liquid contents from the mat into the glass. Usually given free of charge or given to customers asking what their cheapest drink was.

Some guy came in announcing that its their birthday and are expecting me to give them something free and special so I poured the mat into the shot for him and said "Here one mat shot!"